The present invention describes a mechanism for the protection of the correct execution-order of interdependent (interrelated) software applications. In particular, the present invention relates to software license protection for the shielded and timely execution of software applications through the use of tamper-resistant external devices, such as smart cards, USB tokens or other forms for tamper-resistant environments. The present invention also finds its applications in any traditional smart card security application area, such as e-payment, digital rights management (DRM), multi-media protection, authentication, biometry, public-key infrastructure (PKI), encryption-schemes, as an integrated part of special-purpose secure operating systems for tamper-resistant external devices, and the like. The present invention relates to International Application No. PCT/NO96/00171 designating the United States, now U.S. Pat. No. 6,266,416 (Sigbjormsen et al.), which is incorporated by reference herein. This patent describes a system for software license protection through the partial execution of a software application in a tamper-resistant external device.
In multi-processing computer environments, such as for example that provided by a dual-processor environment consisting of an (untrusted) host computer and a (trusted) external, tamper-resistant device such as a smart card or other token, software applications residing on the host computer are sometimes required to make more than one finction call to applications residing on the external device. Software applications which execute on external devices as described herein are also referred to as xe2x80x9capplets.xe2x80x9d Generally speaking, the path of execution of a software application, whether that be a host application or an external device/smart card applet, is critical for the correct functioning of the application. It is important that the execution path is not tampered with.
As a trivial example, an application which converts Celsius temperature degrees into Fahrenheit would need to perform the equation F=9/5*C+32. The multiplication of the Celsius degree by 9/5 must be carried out before the addition by 32. According to the prior art, correct execution order is ensured by letting the function in its entirety be executed in a tamper-proof environment, as an applet, allowing just one function-call entry from the host application to the applet. Since the applet resides on the tamper-resistant smart card, it may not be tampered with, and since the applet has just one entry-point, there is no way the addition operation can possibly be performed before the multiplication operation. Functional atomicity, integrity and correct execution order has been ensured.
If the function F=9*C/5+32 nonetheless was to be split into three applets, as illustrated in FIG. 1; applet 1 as i=9*C, applet 2 as i=i/5 and applet 3 as F=i+32, each with a separate function entry point, then, according to the prior art, no mechanism allows the external device to discover and hinder an adversary from modifying the host application by reversing applet function calls towards the external device, e.g., calling applet 3 before applet 1, or by removing one or several calls, to thereby break the integrity of the compound smart card application.
FIG. 2 shows a scenario where the adversary has removed the call to applet 2, causing the external device to return a corrupt result. Thus, a tamper-proof device does not in general suffice to maintain the correct execution order and overall applet integrity.
A first objective of the present invention is to provide an integrity protection mechanism that allows a smart card application to be split into more than one sub-applications in a safe manner, enforcing correct execution order and application integrity, and to allow the execution environment of the external device to discover any attempts to process applets in an illegal order or manner. This facility opens up new technological possibilities. A second objective of the present invention is to facilitate, for the purposes of software license protection and other application areas indicated above, the selection of software application components suitable and not suitable for execution on the tamper-resistant device, in an unconstrained manner, thereby providing an efficient and highly user-friendly tool for optimization of application security and performance.